Double hard parton-parton interactions are expected to occur frequently in proton-proton (p-p) collisions at the LHC. They can give rise to significant backgrounds to certain rare single scattering (SPS) signals, and are an interesting signal process in their own right. In this thesis, we discuss the theoretical description of the double parton scattering (DPS) cross section in the context of Quantum ChromoDynamics (QCD). After an overview of QCD and an introduction to DPS in Chapter 1, we describe in Chapter 2 a framework for calculating the p-p DPS cross section introduced by Snigirev et al., in which this cross section is expressed in terms of double PDFs D_p {ij}(x_1,x_2,Q_A 2,Q_B 2) (dPDFs). We show that the equal-scale dPDFs are subject to momentum and number sum rule constraints, and use these in the construction of an explicit set of leading order (LO) equal-scale dPDFs (the 'GS09' dPDFs). The leptonic same-sign WW DPS signal obtained using GS09 dPDFs is compared with that obtained using simple factorised forms, and the prospects of observing this signal taking into account SPS backgrounds are analysed. We discuss two ways in which the dPDF framework for describing p-p DPS is deficient in Chapter 3. We discuss interference and correlated parton effects in flavour, spin, colour, and parton type, which are ignored by the dPDF framework. We then study DPS-type graphs in which the parton pairs from both protons have arisen from a perturbative 1->2 branching, derive an expression for the part of such graphs associated with the particles arising from the 1->2 branchings being almost on-shell, and use this to demonstrate that the treatment of these graphs by the the dPDF framework is unsatisfactory. In Chapter 4, we study DPS-type graphs in which the parton pair from only one proton has arisen from a perturbative 1->2 branching. We discover that such graphs contribute to the LO p-p DPS cross section, and that crosstalk between partons in the 'nonperturbatively generated' pair is allowed provided that it occurs at a lower scale than that of the perturbative 1->2 branching in the other proton. The result of this analysis is combined with that of the previous chapter to propose a formula for the LO total DPS cross section, and our proposal is compared with those from other authors. We finish in Chapter 5 with some conclusions and suggestions for further work.

Hadronic collider experiments have played a major role in particle physics phenomenology over the last few decades. Data recorded at the Tevatron at Fermilab is still of interest, and its successor, the Large Hadron Collider (LHC) at CERN, has recently announced the discovery of a particle consistent with the Standard Model Higgs boson. Hadronic colliders look set to guide the field for the next fifteen years or more, with the discovery of more particles anticipated. The discovery and detailed study of new particles relies crucially on the availability of high-precision theoretical predictions for both the signal and background processes. This requires observables to be calculated to next-to-leading order (NLO) in perturbative quantum chromodynamics (QCD). Many hadroproduction processes of interest contain multiple particles in the final state. Until recently, this caused a bottleneck in NLO QCD calculations, due to the difficulty in calculating one-loop corrections to processes involving three or more final state particles. Spectacular developments in on-shell methods over the last six years have made these calculations feasible, allowing highly accurate predictions for final state observables at the Tevatron and LHC. A particular realisation of on-shell methods, generalised unitarity, is used to compute the NLO QCD cross-sections and distributions for two processes: the hadroproduction of W⁺ W^-jj, and the hadroproduction of W⁺ W^-jj. The NLO corrections to both processes serve to reduce the scale dependence of the results significantly, while having a moderate effect on the central scale choice cross-sections, and leaving the shapes of the kinematic distributions mostly unchanged. Additionally, the gluon fusion contribution to the next-to-next-to-leading order (NNLO) QCD corrections to W⁺ W^-j productions are studied. These contributions are found to be highly depen- dent on the kinematic cuts used. For cuts used in Higgs searches, the gluon fusion effect can be as large as the NLO scale uncertainty, and should not be neglected. All of the higher-order QCD corrections increase the accuracy and reliability of the theoretical predictions at hadronic colliders.

This thesis deals with a broad range of aspects that concern the simulation of QCD jet physics by Monte Carlo event generators. Phenomenological work is presented in validating the CKKW approach for merging tree-level matrix elements and parton showers. In the second part the main project is documented comprising the definition, realization and verification of a new QCD colour-dipole cascade. Finally, a new cluster-hadronization model is introduced.

This thesis deals with a broad range of aspects that concern the simulation of QCD jet physics by Monte Carlo event generators. Phenomenological work is presented in validating the CKKW approach for merging tree-level matrix elements and parton showers. In the second part the main project is documented comprising the definition, realization and verification of a new QCD colour-dipole cascade. Finally, a new cluster-hadronization model is introduced.

Le Modèle standard de la physique de particules explique avec beaucoup de succès comment les particules fondamentales interagissent entre elles. Néanmoins, ce modèle n’est pas capable d’expliquer certains défis contemporains du domaine, comme la grande quantité de paramètres libres, le problème de la hiérarchie ou l’identité de la matière sombre. Pour expliquer ces phénomènes, il faut chercher la physique au-delà du Modèle standard. De sorte à satisfaire les contraintes provenant des mesures expérimentales, on peut considérer l’addition d’une seule nouvelle particule au Modèle standard qui se couple à deux quarks, le diquark. Sa contribution aux processus hadroniques offre une opportunité d’obtenir des limites dans ses paramètres à partir des données expérimentales contemporaines. Ceci nous permettrait d’évaluer son potentiel et d’identifier les processus auxquels cette particule peut contribuer. Parmi les états de diquarks possibles, on trouve les diquarks scalaires Dᵘ et Dᵈ, qui possèdent un couplage antisymétrique à des paires de quarks droitiers du type up ou du type down, respectivement. L’obtention des contraintes sur les valeurs de leur masse Mᴰ et leurs couplages aux quarks (x^q, y^q), où q = u, d, est l’objectif central de ce travail. Pour cela, on considère tout d’abord des recherches directes dans la production de dijet, la signature expérimentale des désintégrations de ces particules. Ensuite, on sonde l’influence indirecte du diquark Dᵘ dans les mesures expérimentales de la section efficace de production d’un seul quark top. Les données expérimentales obtenues dans le Grand collisionneur d’hadrons (LHC) pour ces processus à haute énergie sont utilisées pour comparer les prédictions théoriques de ces modèles et quantifier leur viabilité en fournissant de nouvelles contraintes sur ses paramètres. On est capable d’améliorer les contraintes trouvées dans la littérature de |xᵘ| ≤ 14,4 à |xᵘ| ≤ 0,13–0,15 pour Dᵘ et de |xᵈyᵈ| ≤ 0,022 avec |yᵈ| ≤ 0,17 à |xᵈ| ≤ 0,15–0,17 pour Dᵈ, les deux avec une masse de Mᴰ = 600 GeV. De plus, l’analyse des données pour la production d’un seul quark top montre que la réduction de l’espace de phase des pᵀ peut améliorer encore plus ces contraintes. / The Standard Model of particle physics explains with great success how fundamental particles interact. However, this model cannot explain some contemporary challenges of the domain, such as the large number of free parameters, the hierarchy problem or the identity of dark matter. To explain these phenomena, we need to search for physics beyond the Standard Model. In order to satisfy the existing constraints from experimental measurements, we can consider the addition of a single new particle to the Standard Model that couples to two quarks, a diquark. Its contribution to hadronic processes offers an opportunity to set limits on its parameters from contemporary experimental data. This would allow us to evaluate their potential and identify processes to which they can contribute. Among the possible diquark states, we find the scalar diquarks Dᵘ and Dᵈ, which have an antisymmetric coupling to pairs of right-handed up-type or down-type quarks, respectively. To obtain constraints on their mass Mᴰ and on their couplings to quarks (x^q, y^q), where q = u, d, is the main goal of this work. In order to do so, we start by considering direct searches in the production of dijet, the experimental signature of the decay of these particles. Afterwards, we probe the indirect influence of the Dᵘ diquark to the experimental measurements of the single-top-production cross section. Recent data for both of these processes from the Large Hadron Collider (LHC) is used to compare the theoretical predictions of these models and quantify their viability by providing new constraints on its parameters. We are able to improve the constraints found in the literature from |xᵘ| ≤ 14.4 to |xᵘ| ≤ 0.13–0.15 for Dᵘ and from |xᵈyᵈ| ≤ 0.022 to |yᵈ| ≤ 0.17 to |xᵈ| ≤ 0.15–0.17 for Dᵈ, both with masses of Mᴰ = 600 GeV. Other than that, the analysis of the data from single top production shows that the reduction of the pᵀ space can further improve these constraints.

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